The invention relates to a method of dewatering a fibrous web in a drying section, preferably a paper web, cellulose web or cardboard web in a paper machine, the drying section comprising a number of drying cylinders, by which an inlet and outlet nip are formed when running the fibrous web, the drying section comprising drying devices for decreasing the moisture content of the fibrous web.
The invention also relates to a drying section for dewatering a fibrous web, the drying section preferably being intended to dry fibrous webs in a paper machine, such as paper webs, cellulose webs or cardboard webs, and the drying section comprising a number of drying cylinders, by which an inlet and outlet nip are formed when running the fibrous web, and a blow device for blowing air in the proximity of the surface of the fibrous web.
A drying section is an essential part of a paper machine. The drying section serves to decrease the initially high moisture content of a paper web (or other fibrous web) so that its dry content increases to about 90 to 97% when it leaves the drying section. Paper machines have become increasingly fast, and this has led to the need to make their drying sections correspondingly longer. In practice, this means that the drying section may be a voluminous part of the paper machine having a considerable length, for example 80 meters, and comprising a plurality of drying cylinders.
It is naturally desirable to accomplish a drying section so as to further increase the travel speed of the fibrous web, i.e. the speed of the paper machine, from what is customary, without making the drying section too complicated and large. Blowing hot air towards a paper web is a means for decreasing the length of the drying section. In spite of this, the drying section is a very complicated construction.
In this context, the concepts xe2x80x98humid airxe2x80x99 and xe2x80x98airxe2x80x99 refer to a combination of dry air and water vapour.
The object of the present invention is to provide a significant improvement to the drying section in machines processing fibrous webs, paper machines, for example, in such a way that the speed of the machines can be considerably increased and, simultaneously, the number of drying cylinders can be decreased and the construction and operating costs lowered.
This object is achieved by the method of the invention, characterized by guiding the fibrous web wirelessly against a porous drying cylinder, heating the fibrous web by radiation heat directed towards the fibrous web and the drying cylinder, and providing by means of lateral blowing a humid airflow which sweeps in the machine direction along the surface of the fibrous web.
This object is achieved by the drying section of the invention, characterized in that the dewatering devices comprise
a porous drying cylinder against which the fibrous web is arranged to be guided wirelessly,
a heating means arranged at a distance from the shell surface of the porous drying cylinder and directed towards the fibrous web and drying cylinder, for heating the fibrous web, the heating means being based on heating with rays having a wavelength which activates water molecules, and
the blow device comprising lateral blow devices arranged to provide a humid airflow which sweeps along the surface of the fibrous web.
The heating employs electromagnetic waves, preferably IR rays (infrared rays) or microwaves.
The lateral blow devices are preferably arranged to blow in a direction opposite to that of the movement of the fibrous web, resulting in more rapid dewatering.
When the heating means is of a type based on gas energy, and the flame heat is guided without hindrance against the fibrous web, particularly good heat transfer and a dewatering process having a high efficiency are achieved. Guiding heat without hindrance means herein that a heat absorbing material (for example a ceramic material) is only partially present between the fibrous web and the flame to achieve the desired wavelength spectrum which thus allows optimal heat distribution and efficiency.
To further accelerate the dewatering process, blow devices are preferably used for blowing humid air along the inside of the drying cylinder, the air pressure being held lower than the air pressure acting above the fibrous web. This allows moisture to be conveyed from the fibrous web through the drying cylinder, too. This embodiment enables moisture removal from both sides of the fibrous web. Part of the discharge air can also be used to heat the inlet air.
Preferred embodiments of the invention are disclosed in the attached claims 3 to 16.
The use of heating means based on heating by electromagnetic waves, for instance infrared rays, allows the surface temperature of a fibrous web to be increased even though the fibrous web is conveyed at a high speed, for example 20 to 40 m/s. The blow devices allow the moisture to be freed from the fibrous web and discharged rapidly.
The most important advantage of the invention is a more rapid dewatering process, which can be achieved at the same time as the construction and operating costs can be kept lower than usual. The number of drying cylinders can be substantially reduced, to about ⅕ of the number of drying cylinders in conventional cylinder drying. The costs per kilo of evaporated water are substantially reduced, compared with conventional paper machines, by 20 to 40%. The heat energy based on electromagnetic waves can be transferred effectively, while the remaining heat energy is transferred parallel to the radiation energy in the form of forced convection and guidance through the porous cylinder. The drying section of the invention enables an even dry content and an end product of high and uniform quality. The uniform quality is guaranteed by the fibrous web being attached during the dewatering process and by double-sided evaporation and by subjecting the fibrous web only to very slight mechanical stress. An even dry content in the end product is ensured simply by a number of IR heating units on the last drying cylinders of the drying section.